1. Field of the Invention
The present invention relates to an image data binarization technique, and more particularly, to an image binarization technique upon output to a printer incorporated in a digital copier, a multi-function apparatus or the like.
2. Description of the Related Art
Conventionally, as an image binarization technique used in an image output apparatus such as a printer, an error diffusion method is known. As characteristic features of the error diffusion method, the occurrence of moiré is prevented and gradation reproducibility is excellent. Further, in the error diffusion method, the texture structure is smaller in comparison with conventional systematic dithering and density pattern method, and the gradation is locally preserved. Accordingly, satisfactory image quality can be obtained in character and line art, and gradation images.
However, the error diffusion method has some problems. One of the problems is that there is a texture structure based on dot anisotropy in highlight and shadow portions. In such highlight and shadow portions, as the dispersion of black dots (or white dots) is poor and not isotropic, the black dots are arranged in uniflow manner, which degrades image quality. The occurrence of dot anisotropy depends on an error diffusion matrix. The dot anisotropy also occurs in a popularly-used error diffusion matrix by Floyd & Steinberg, which is comparatively simple and realized by a small hardware scale.
Another problem is a “sweep out texture phenomenon”. This phenomenon occurs in an image with a drastic gradation change like black-to-white (or white-to-black) change, by delayed dot generation. The sweep out texture phenomenon is also referred to as a dot delay phenomenon (retardation). When this phenomenon occurs, pseudo outlines occur in highlight and shadow portions, which cause a serious problem in image quality. Especially in a CG image based on image data without noise component, the pseudo outlines are conspicuous.
These two problems particularly depend on the shape of the error diffusion matrix and contents of input image data. Accordingly, it is necessary to carefully select an error diffusion matrix. However, as described later, the “anisotropy phenomenon” and the “sweep out texture phenomenon” are in trade-off relation, and it is difficult to solve the both problems only by selection of error diffusion matrix.
As a solution to these problems, a method for forced dot determination without error diffusion matrix has been proposed (N. Karito: A Halftoning Method Using Circular Cell, NIHON GAZO GAKKAISHI, 46, 103-106 (2007))(Document 1). The Document 1 discloses obtaining an accumulated value of unbinarized neighboring image data of a current pixel, and obtaining the number of blacken dots and a cell size (1 halftone region). In this method, the obtained number of halftone lines can be controlled by controlling a search range, however, as a search is made through multi-bit unbinarized pixels, the capacity of an image memory is increased. Further, when a search is moved from a highlight portion to a shadow portion, a change from a black dot to a white dot cannot be made.
On the other hand, a Document 2, G. Marcu; “Error diffusion algorithm with output position constraints for homogeneous highlight and shadow dot distribution”, Journal of Electronic Imaging, Vol. 9(1), pp 46-51, discloses searching for an already-binarized pixel in an image highlight and shadow portions, to detect presence/absence of dot in the neighborhood of a current pixel, and based on the result of detection, binarizing the current pixel. According to the technique disclosed in the Document 2, in comparison with the method in the above-described Document 1, the search is made for an already-binarized pixel, thereby a memory capacity only for a 1-bit plane is required. That is, the memory capacity is smaller. Further, in this method, as an error diffusion method is used in an intermediate portion between the highlight and shadow portions, the transition from a black dot to a white dot in the intermediate density portion can be smoothly performed by dot connection based on the error diffusion method.
However, in the Document 2, a change from a highlight portion to an intermediate portion or from an intermediate portion to a shadow portion cannot be smoothly made, and a pseudo outline occurs in the boundary. Further, the size of generated dots is fixed, which may not match various printer characteristics to be described later.
In a digital printing apparatus based on electrophotograpy, variations in special frequency in a print image cannot be handled due to a nonlinear characteristic of an electrophotographic process. This weak point comes from nonlinearity at an exposure process to diselectrify electric charge on the electronically-uniformly charged surface of an electrostatic drum of OPC, amorphous silicon or the like, by light beam scanning using laser or the like, and complexity of electrophotographic process including developing, transfer and fixing. For example, a minute 1 dot cannot be printed without difficulty, but actually a several dot-cluster can be printed. Dots existing with a minute distance therebetween are attached or moved away by toner movement due to the distance.
In this situation, in a so-called disperse type method in an FM halftone screen, high image quality cannot be obtained in the electrophotographic printing without difficulty, while in a cluster type method for dot concentration, stable gradation reproducibility can be achieved.